1. Field of the Invention
This invention relates to low expansion porcelain compositions, and in particular to relates to low expansion, low maturing temperature porcelain compositions useful for the fabrication of dental restorations.
2. Description of the Related Art
Porcelain materials are used in dentistry in order to obtain natural-looking dental restorations. Porcelains are highly desirable for this purpose since they can be colored to closely resemble the teeth they must replace, resist degradation inside the oral cavity, and remain biocompatible even after years of continuous contact with mammalian tissue. Restorations may be classified as either porcelain-fused-to-metal (PFM) or as all-ceramic restorations.
Typically, PFM restorations are fabricated by applying a dental porcelain powder in aqueous slurry to a metal alloy framework and firing the porcelain at high temperature to form a tight, impervious porcelain layer having the appearance of natural dentition. Those skilled in the art recognize that it is important that the firing temperature of the porcelain must be compatible with the material used for the metal framework. For example, titanium and titanium alloys require overlay porcelain having firing temperatures below the temperature at which the alpha crystalline structure transforms to the less useful beta crystalline structure. It is further important that the thermal expansion behavior of the porcelain be compatible with the thermal expansion behavior of the metal so that no stress cracks are produced in the porcelain layer due to thermal expansion mismatch stress occurring during firing and cooling down.
Today, there is an increasing trend in dentistry toward the use of ceramic cores in lieu of metal alloy frameworks to provide all-ceramic dental restorations. Where a ceramic is employed as the core of a dental restoration, any porcelain applied to the ceramic framework or coping must also possess a coefficient of thermal expansion (CTE) that is compatible with that of the ceramic in order to avoid production of stress cracks in the core and/or porcelain.
Metal alloys and ceramics employed in the manufacture of dental restorations have typically possessed moderately high coefficients of thermal expansion, in the range from about 13xc3x9710xe2x88x926/xc2x0 C. to about 17xc3x9710xe2x88x926/xc2x0 C. Many porcelain compositions are known in the art which are thermally compatible with these moderately high expansion core materials and provide smooth, fused glassy surface on the resulting dental restorations. However, few porcelain compositions are suitable for use with low expansion alloys and ceramics, i.e., those alloys and ceramics having coefficients of thermal expansion in the range of about 7xc3x9710xe2x88x926/xc2x0 C. to about 13xc3x9710xe2x88x926/xc2x0 C.
Accordingly, there remains a need in the art for porcelain compositions thermally compatible with low expansion core materials; having maturing temperatures below about 850xc2x0 C.; which are chemically and thermally stable; and which provide a smooth, non-abrasive surface when applied to low expansion alloys and porcelains.
The above mentioned drawbacks and disadvantages are overcome or alleviated by a dental porcelain composition comprising an amorphous glass phase with a maturing temperature less than about 850xc2x0 C., wherein the amorphous glass phase, in one embodiment, comprises:
The above discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description.
A dental porcelain composition which is low fusing and suitable for use with titanium, titanium alloys and ceramic cores, and which provides an extremely smooth surface for dental restorations, comprises an amorphous glass phase with a maturing temperature less than about 850xc2x0 C. and a coefficient of thermal expansion (25xc2x0 C. to 500xc2x0 C.) of about 7xc3x9710xe2x88x926/xc2x0 C. to about 11xc3x9710xe2x88x926/xc2x0 C. The compositions find particular utility as overlay porcelains for veneers, single and multi unit restorations such as dental crowns and bridges (fixed partial dentures), inlays and onlays.
The dental porcelain composition comprises, on a weight percent basis:
The dental porcelain compositions are amorphous glasses that mature at a temperature consistent with the thermal stability temperature of low expansion porcelain cores and alloy frameworks. That is, the porcelain forms a chemical bond with the core and has a thermal expansion value within about 2xc3x9710xe2x88x926/xc2x0 C. of that of the core. Components such as Li2O, BaO, F, TiO2, ZnO and SnO2 are added to these glasses to provide wettability and good bonding to the cores used with these porcelains. ZnO and TiO2 are particularly useful if the porcelain composition is to be used in conjunction with titanium and titanium alloys.
The porcelain compositions are chemically and thermally stable and have sufficient viscosity at firing temperature to maintain the required shape of dental restorations mimicking that of tooth anatomy. The porcelain compositions are fired at temperatures not exceeding about 850xc2x0 C. The porcelain composition fires to nearly 100% of theoretical density, thus forming a tight impervious surface necessary in the oral environment.
A preferred feature of the present composition is a combination of Al2O3, B2O3, and MgO effective to achieve low maturing temperature, while at the same time maintaining low thermal expansion and high chemical durability. While B2O3 often lowers thermal expansion and maturing temperature, it can simultaneously decrease the chemical durability of porcelain if it comprises more than about 3-4 wt % of the total composition. To lower expansion and maturing temperature while maintaining high chemical durability, B2O3 is therefore preferably used in combination with Al2O3 and MgO.
The porcelain compositions can be prepared by melting together sufficient precursor components to yield the compositions shown in the above table. Suitable precursors include silica, alumina, boric acid, feldspar, calcium carbonate, sodium carbonate, potassium carbonate, lithium carbonate or lithium fluoride, or if desired, the actual oxides, blended in proportion to yield the compositions shown in the above table.
The preparation of such materials is well known in the art. After the materials are blended, preferably in finely divided powder form such as powder sufficiently fine to pass through a 200 mesh screen (Tyler series), the precursors and/or oxides are heated to a temperature of at least about 1100xc2x0 C., and preferably to at least about 1230xc2x0 C., in a crucible to form a glass.
The molten glass may then be quenched in water, dried, and ground in a ball mill, to provide the porcelain material in the form of a powder.
It is preferred that the powder is ground finely enough so that it will pass through a 200 mesh screen (Tyler series). Opacifiers, pigments and fluorescing agents are then added to this powder in the amount of up to about 5 wt % for body and incisal porcelain compositions and up to 30 wt % for opaques.
The properties of the porcelain composition can be adjusted by applying the following well-known principles. Within the ranges of component proportions set forth in the above table, the coefficient of thermal expansion can be increased, if desired, by decreasing the proportion of SiO2 and/or B2O3 and/or by increasing the proportion of the alkali metal oxides. The fusion point can be reduced by increasing the proportion of B2O3, CaO, and/or the alkali metal oxides. As between the two alkali metal oxides, an increase in the Na2O:K2O ratio may lower the fusion point. However, when complex mixtures of alkali oxides are used, the so-called mixed alkali phenomenon affects the properties of the composition in a non-linear fashion. It is well within the skill of the porcelains art to apply these principles to make fine adjustments to the thermal expansion coefficients and fusion temperatures.
If desired, in order to achieve proper aesthetics, one or more layers of the porcelain composition can be applied over the core with each layer being separately fired. Thus, for example, an opaceous layer containing an opacifying agent such as TiO2, SnO2, Al2O3, ZnO, CeO2, ZrO, ZrSiO4 and the like can be applied over the core and fired. Thereafter, or in lieu thereof, a stain layer can be applied containing one or more conventional pigments such as vanadates, manganates, chromates, or other transition metal compounds, to tint the stain layer to the desired shade. The opaceous and/or stain layer can then be overcoated (after sequential firing) with a translucent layer of the porcelain composition of the present invention. In this manner, special effects can be obtained, e.g., a different shade at the tip of the restoration than at the gingival area. The layers are applied to the core in the usual manner, as by applying a paste of the porcelain powder in water over the core, shaping to the desired configuration, and then firing.
In an alternative embodiment, amorphous glasses in the form of powder (a frit) are mixed with a second glass flit, glass-ceramic frit and/or crystalline filler to modify the firing temperature and thermal expansion. Suitable crystalline fillers can be mullite or alumina particles to lower the thermal expansion to about 6 to about 7xc3x9710xe2x88x926/xc2x0 C. Preferably the average particle size of the crystalline filler is less than about 10 microns, and more preferably, less than about 3 microns. The second frit can be a lower maturing temperature glass to lower the composition""s maturing temperature and increase the expansion to about 11xc3x9710xe2x88x926/xc2x0 C.
Preferred core materials include ceramics comprising lithium disilicate glass ceramics, zirconia, and micaceous glass ceramics, as well as other ceramic cores with thermal expansions in the range of about 7 to about 13xc3x9710xe2x88x926/xc2x0 C. Suitable metal and alloy cores include those based on Ti and Ti alloys.